(a) Field of the Invention
The present invention relates to a variable focal length lens system and, more particularly, to a variable focal length lens system suitable for the use with compact cameras such as lens shutter cameras.
(b) Description of the Prior Art
Compact cameras such as lens shutter cameras are widely used because they are handy and very convenient for carrying. In recent years, compact cameras have made a remarkable advance in various points, i.e., enriched additional functions such as built-in stroboscopic lamp, automatic focus adjustment, automatic film loading, etc., and automatization of various operations, and have become more and more convenient to use. To make the range of use of compact cameras still wider, those provided with variable focal length lens system as photographing lens systems have appeared in addition to conventional compact cameras provided with lens systems with fixed focal length.
Lens systems disclosed, for example, in Japanse published unexamined patent applications Nos. 215620/83 and 48009/85 are known as the above-mentioned kind of variable focal length lens systems. The known lens system disclosed in Japanese published unexamined patent application No. 215620/83 comprises a first lens unit comprising four lenses and having positive refractive power, a second lens unit comprising two lenses and a third lens unit comprising a positive single lens and is arranged to vary the focal length of the lens system as a whole by keeping the third lens unit fixed in relation to the image surface and moving the first and second lens units along the optical axis. The focal length varying ratio of said known lens system is large, i.e., about 2 times. However, said known lens system comprises a large number of lenses. Besides, as the second lens unit to be moved has a large diameter, a wide space is needed at the time of moving. Therefore, it is difficult to make the lens system as a whole compact, and the camera tends to become large.
On the other hand, the variable focal length lens system disclosed in Japanese published unexamined patent application No. 48009/85 comprises a first lens unit comprising three lenses and having positive refractive power and a second lens unit comprising a negative single lens and is arranged to move said two lens units along the optical axis. Therefore, the number of lenses constituting the lens system is comparatively small.
However, also in case of said known lens system, the second lens unit to be moved has a very large diameter and needs a large space for the purpose of moving. Consequently, it is unaovidable to reserve a large space for the lens system in the inside of the camera and, therefore, it is also difficult to make the camera compact.